An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a PWM (Pulse Width Modulation) control.
A general configuration of the conventional electric power steering apparatus (EPS) will be described with reference to FIG. FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Moreover, the column shaft 2 is provided with a steering angle sensor 14 for detecting a steering angle θ. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 on the basis of a voltage control value Vref obtained by performing compensation and so on with respect to the calculated current command value.
A CAN (Controller Area Network) 40 for transmitting/receiving various information about the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, a non-CAN 41 for transmitting/receiving communications, analog/digital signals, radio waves, etc. except for the CAN 40 can also be connected to the control unit 30.
In such an electric power steering apparatus, the control unit 30 mainly comprises a CPU (also including an MPU, an MCU, or the like), and for example, general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vel and by means of an assist map or the like. The calculated current command value Iref1 is added to a compensation signal CM from a compensating section 34 for improving characteristics in an adding section 32A, an added current command value Iref2 is inputted into a current limiting section 33 so that a maximum current is limited. A current command value Irefm that the maximum current is limited, is inputted into a subtracting section 32B, and a subtraction of a motor current detection value Im from the current command value Irefm is performed.
A subtraction result I (=Irefm-Im) of the subtracting section 32B is PI-controlled in a PI control section 35. The PI-controlled voltage control value Vref is inputted into a PWM control section 36 and synchronized with a carrier signal CF so that the duty is calculated. Furthermore, the motor 20 is PWM-driven through an inverter 37 by a PWM signal that the duty is calculated. The motor current value Im of the motor 20 is detected by a motor current detecting means 38 and inputted into the subtracting section 32B to be fed back.
The compensating section 34 adds a detected or estimated self-aligning torque (SAT) 343 to an inertia compensation value 342 in an adding section 344, further adds a convergence control value 341 to an addition result of the adding section 344 in an adding section 345, and then inputs an addition result of the adding section 345 into the adding section 32A as the compensation signal CM to improve the characteristics.
In an electric power steering apparatus comprising a torsion bar, it is necessary to detect angles at a plurality of positions, for example, sensors shown in FIG. 3 are mounted on the column shaft 2, and various detection signals are outputted. That is to say, a Hall IC sensor 21 as an angle sensor and a 20° rotor sensor 22 as an input side torque sensor are mounted on an input shaft 2A of a steering wheel 1 side of the handle shaft 2. The Hall IC sensor 21 outputs an AS_IS angle θh of 296° period, and the AS_IS angle θh is inputted into a steering angle calculating section 40. The 20° rotor sensor 22 that is mounted on the steering wheel 1 side than a torsion bar 23, outputs TS_IS angles θs1 (main) and θs2 (sub) of 20° period, and the TS_IS angle θs1 is inputted into the steering angle calculating section 40. Further, a 40° rotor sensor 24 as an output side torque sensor is mounted on an output shaft 2B of the handle shaft 2, TS_OS angles θr1 (main) and θr2 (sub) are outputted from the 40° rotor sensor 24, and the TS_OS angle θr1 is inputted into the steering angle calculating section 40. The steering angle calculating section 40 calculates a steering angle θab being an absolute value on the basis of the AS_IS angle θh, the TS_IS angle θs1 and the TS_OS angle θr1 to output.
FIG. 4 shows FIGS. 4(A), 4(B) and 4(C) show one example of signal period of the detection signal of each sensor. FIG. 4(A) shows the signal period (296°) of the AS_IS angle θh being the detection signal from the Hall IC sensor 21, FIG. 4(B) shows the signal period (20°) of the TS_IS angle θs1 being the detection signal from the 20° rotor sensor 22, and FIG. 4(C) shows the signal period (40°) of the TS_OS angle θr1 being the detection signal from the 40° rotor sensor 24. “0” point adjustments of these three sensors are adjusted by performing calibration at assembling.
In such an electric power steering apparatus, recently, reliability improvement is further required and the redundancy of apparatuses and parts are carried out. As such a redundancy apparatus, for example, there is a detection signal processing method disclosed in Japanese Published unexamined Patent Application No. H6-32240 A (Patent Document 1), and this detection signal processing method is applied to a steering system for automobile. Further, a physical quantity sensor comprising a second conversion processing section connected to output terminals for converting a first output signal outputted from a first sensor element and a second output signal outputted from a second sensor element into a second physical quantity that the second conversion processing section is disposed within a second package, is disclosed in the publication of Japanese Patent No. 4863953 (Patent Document 2).